# Assignment: Explainable First-order Tableau

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COL703: Logic for Computer Science
Assignment: Explainable First-order Tableau
Problem Statement : In this assignment, you have to write a program which given a finite (possibly empty)
list of closed formulae Φ = {φ1, . . . , φn} and another closed formula φn+1, constructs an analytic tableau to
check if φn+1 is a logical consequence of Φ. You have to construct the First Order Tableau using unification.
If ψ is not a logical consequence of Φ then your program should run forever (or until you get a segmentation
fault or it is interrupted).
If Φ = {φ1, . . . , φn} |= ψ, then a finite closed tableau should be the output.
Input : The input is a file arg.sml containing an argument as defined int eh signature given below.
Output : The output in case the argument is correct, is a file tableau.dot in dot format. The first-order
tableau is “explainable” if it also has the dotted blue arrows and the red links as in the Hypernotes along with
the unification remarks.
signature FOL =
sig
datatype term = VAR of string
| FUN of string * term list
| CONST of string (* for generated constants only *)
datatype Pred = FF (* special constant for closing a tableau path *)
| ATOM of string * term list
| NOT of Pred
| AND of Pred * Pred
| OR of Pred * Pred
| COND of Pred * Pred
| BIC of Pred * Pred
| ITE of Pred * Pred * Pred
| ALL of term * Pred
| EX of term * Pred
datatype Argument = HENCE Pred list * Pred
fun mktableau: Pred list * Pred -> unit (* outputs file “tableau.dot” in dot format *)
exception NotVAR (* Binding term in a quantified formula is not a variable *)
exception NotWFT (* term is not well-formed *)
exception NotWFP (* predicate is not well-formed *)
exception NotWFA (* argument is not well-formed *)
exception NotClosed (* a formula is not closed *)
end
2. The predicate constructor FF signifies the end of a closed path in the tableau.
3. The first occurrence of a function/predicate symbol determines its arity. A term/predicate is not wellformed if it has different arities in different occurrences in the argument.
4. A constant a is written as FUN (‘‘a’’, []) if it is part of the input argument. However you may have
to generate new constants c which are then written as CONST ‘‘c’’.
5. An atomic proposition (parameterless) p is written ATOM (‘‘p’’, []).
6. The consructors ALL and EX stand for ∀ and ∃ respectively. Obviously all well-formed quantified formula
should be of the form ALL (VAR ‘‘string’’, phi) or EX (VAR ‘‘string’’, phi) where phi denotes
a predicate.
Sample dot file source code (without colours and fonts). By running the following commands
\$ /usr/bin/dot2tex sample.dot > sample.dot.tex
\$ pdflatex sample.dot.tex
we obtain a file sample.dot.pdf which displays the full tableau (Try it!).
digraph{
nodesep = 0.5;
ranksep = 0.35;
node [shape=plaintext];
0 [texlbl=”\underline{0. \$\exists x[\neg p(f(g(x)))]\$ }”];
1 [texlbl=”\underline{1. \$\forall y[p(y)]\$ }”];
2 [texlbl=”\underline{2. \$\neg p(f(g(a)))\$ }”];
3 [texlbl=”\underline{3. \$p(f(g(a)))\$ }”];
4 [texlbl=”\underline{4. \$\bot\$ }”];
subgraph dir
{
0 -> 1;
1 -> 2;
2 -> 3;
3 -> 4;
}
subgraph ancestor
{
edge [dir=back, color=blue, style=dashed]
0->2;
1->3 [label=”Unify (1,2)”];
}
subgraph undir
{
edge [dir=none, color=red]
2 -> 3
}
}
Unify (1,2)
0. ∃x[¬p(f(g(x)))]
1. ∀y[p(y)]
2. ¬p(f(g(a)))
3. p(f(g(a)))
4. ⊥

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